The present invention relates to generally an open-circuit hydraulic actuating system, and more particularly a hydraulic actuating system of the type especially adapted for use with the load whose direction is reversed during operation.
When the load, which is initially stationary, is accelerated to a desired speed which is maintained for a predetermined time so that the load may accomplish a desired work and then the load is decelerated and stopped stationary; that is, when it is desired to control the speed of the load when the direction of the load is reversed during operation, it is imperative that the acceleration stage or stroke may be shifted to the constant speed stage or stroke as immediately and smoothly as possible; that the constant speed stage or stroke may be shifted to the deceleration stage or stroke as immediately and smoothly as possible; and that the speed of the load must be maintained as constant as possible in the constant speed stage or stroke so that the smooth operation may be ensured; the hydraulic system may be made compact in size; and the operating efficiency may be improved. One of the well known hydraulic systems which may satisfy the above requirements is a closed circuit hydraulic system in which a variable displacement hydraulic pump and a fixed displacement hydraulic motor are intercommunicated through tow main circuits. The first advantages of the hydraulic system of the type described is that the speed of the load may be infinitestimally controlled, and the second advantage is that the maximum speed of the load may be easily controlled by controlling the discharge of the hydraulic pump. The third advantage is that the power loss in the deceleration stage or stroke is very small even when a relief valve is actuated. Therefore, one may consider that a closed circuit hydraulic actuating system consisting of a variable displacement hydraulic pump and a fixed displacement hydraulic motor could satisfy the above requirements, but this is not true. In the closed circuit hydraulic actuating system, the speed control and reversal in direction of the load must be accomplished by the hydraulic pump. Therefore the hydraulic pump must satisfy the following requirements (a) that the discharge may be infinitestimally controlled, (b) that the direction of the discharge may be freely selected or reversed; and (c) that it must produce the retarding force. In order to satisfy the above requirements, very expensive hydraulic pumps such as axial plunger pumps must be used, but result is the increase in the cost and the excessive noise in operation. Furthermore the zero-point control (that is, the control of the point at which the discharge is zero) of the hydraulic pump is extremely difficult so that it is difficult to bring the load completely stationary. Moreover, during the operation at a low speed with a small capacity, hunting phenomenon occurs in a flushing valve unit.
Therefore, it is very advantageous to provide an open circuit hydraulic actuating system which may substantially overcome the above defects and may control the speed of the load under the operating conditions described above. For this purpose, there has been proposed an open circuit hydraulic actuating system which consists of a fixed displacement pump, a hydraulic motor and a directional control valve. In this system, a flow control valve whose operation is controlled in response to the operation of the directional control valve is inserted between the directional control valve and the hydraulic motor for controlling the flow rate of the return flow from the hydraulic motor, thereby controlling the speed of the load. There has been also proposed an open circuit hydraulic actuating system in which a flow control valve whose operation is controlled in response with the operation of a flow control valve is inserted between a hydraulic pump and the directional control valve for controlling the flow rate of the hydraulic liquid to be supplied to the hydraulic motor, and a counter balance valve is inserted between the flow directional valve and the hydraulic motor for preventing the free running of the hydraulic motor. However, in both of the hydraulic systems of the type described above, the back pressure is generated at the discharge port of the hydraulic motor even in the accleration and constant speed stages or strokes in which the direction of the load is positive so that the circuit efficiency is comparatively low. Furthermore, unless the speed of the load rises in excess of the discharge of the hyraulic pump, the discharge pressure of the hydraulic pump rises to the set pressure of the main relief valve even when the load is less so that the power loss is considerably increased. Especially in the deceleration stage or stroke in which the direction of the load is negative, the pressure on the return or counter side rises to an excessively high level not only due to the pressure produced by the load but also due to the pressure on the supply side which rises to the set pressure of the main relief valve. Therefore, the withstanding pressure of the hydraulic equipment and pipe lines must be determined based upon this excessively high pressure level with the result of the inevitable increase in cost.